1. Field of the Invention
This invention relates to improvements in mass data storage devices, and more particularly to frequency compensated amplifiers and methods for providing frequency compensation to amplifiers thereof to reduce a second order frequency response of a magneto-resistive preamplifier used therein.
2. Relevant Background
Mass data storage devices include tape drives, as well as hard disk drives that have one or more spinning magnetic disks or platters onto which data is recorded for storage and subsequent retrieval. Hard disk drives may be used in many applications, including personal computers, set top boxes, video and television applications, audio applications, or some mix thereof. Many applications are still being developed. Applications for hard disk drives are increasing in number, and are expected to further increase in the future.
Recently, hard disk drive manufacturers have begun to use magneto-resistive (MR) heads to provide the required sensitive magnetic transducer for reading data from the spinning disk of the drive. MR heads efficiently convert medium magnetization changes into sufficiently high current or voltage with a minimum amount of noise, detect signals at high densities with a negligible loss in signals, and are cost-effective. Moreover, MR-sensor technology is extendable to very high disk drive densities. The term xe2x80x9cmagneto-resistivexe2x80x9d refers to the change in resistivity of metals in the presence of a magnetic field.
Although in the past, MR heads were thought of as having a very low inductance, recently the inductance of the heads have become a concern as the data densities of the drive have increased. The MR heads, including their inductive components have been modeled in various ways. For example, a circuit model 5 having a differential magneto-resistive head (MR) 10 and its associated preamplifier 12 for use in a hard-disk drive application is shown in FIG. 1. The MR recording head 10 is represented by a resistor 14 designated as Rmr. In the circuit model of FIG. 1, the MR head is biased by a bias voltage source 16, and the parasitic inductive elements 18 and 20 are shown in series between the resistor 14 and bias voltage source 16. The resistor 14 is capacitively coupled by parasitic capacitors 22 and 24 to the differential preamplifier 26 to provide the differential output voltage Vout.
A similar circuit model 28 that is biased by a current source 30 is shown in FIG. 2. In the circuit 28, the resistor 14 of the MR head is connected in parallel with the current source 30, and is capacitively coupled by the parasitic capacitances 22 and 24 to the differential amplifier by op-amps 32 and 34. The output of the op-amps 32 and 34 provide the differential input to the differential preamplifier 12. Other combinations of the head bias and sense schemes are known. For example, V-bias/I-sense and I-bias/V-sense schemes can easily be created.
Using the differential V-bias/V-sense amplifier configuration 5 of FIG. 1 for the following illustration, the circuit can be described by the equations                                           Δ            ⁢                          xe2x80x83                        ⁢                          V              S                                            Δ            ⁢                          xe2x80x83                        ⁢                          V              B                                      =                              Δ            ⁢                          xe2x80x83                        ⁢                          R              mr                                            R            mr                                              EQ        ⁡                  (          1          )                    
and
Vout=Axc3x97xcex94Vsxe2x80x83xe2x80x83EQ(2)
where a change in Rmr (xcex94Rmr) generates a signal voltage of xcex94Vs.
However, as mentioned, in reality, the MR head is not a pure resistor. The head contains the parasitic inductances (Lp), and the parasitic capacitances (Cp) in addition to the intended resistance Rmr.
The MR heads, themselves, have been modeled in industry. For example, an electrical schematic diagram of a full model 38 of a magneto-resistive head of the type used in mass data storage devices is shown in FIG. 3, and an electrical schematic diagram of a simple model 40 of a magneto-resistive head of the type used in mass data storage devices is shown in FIG. 4. The full model 38 is essentially modeled as a transmission line of inductors 42-45 and capacitors 47-49, with the resistor 14 of the head at one end and the connection to the preamplifier at the other end. The simple model 40 includes only two lumped inductors 50 and 52 and a single lumped capacitor 54.
With reference to the simple model 40 of FIG. 4, it can easily be shown that                                           Δ            ⁢                          xe2x80x83                        ⁢                          Vs              xe2x80x2                                            Δ            ⁢                          xe2x80x83                        ⁢            Vs                          =                              1                          2              xc3x97                              L                p                            xc3x97                              C                p                                                                        s              2                        +                          (                              s                xc3x97                                                      R                    mr                                                        2                    xc3x97                                          L                      p                                                                                  )                        +                          1                              2                xc3x97                                  L                  p                                xc3x97                                  C                  p                                                                                        EQ        ⁡                  (          3          )                    
where xcex94Vs is the signal voltage generated by xcex94Rmr, while xcex94Vsxe2x80x2 is the resultant signal voltage applied to the preamplifier input.
Note that EQ(3) describes a second-order system with                               ω          n                =                  1                                    2              xc3x97                              L                p                            xc3x97                              C                p                                                                        EQ        ⁡                  (          4          )                    
and                     Q        =                              1                          R              mr                                xc3x97                                                    2                xc3x97                                  L                  p                                                            C                p                                                                        EQ        ⁡                  (          5          )                    
Therefore,                               V          out                =                  A          xc3x97          Δ          ⁢                      xe2x80x83                    ⁢                      V            S            xe2x80x2                                                        xe2x80x83                ⁢                  EQ          ⁡                      (            6            )                                                  =                  A          xc3x97                                    1                              2                xc3x97                                  L                  p                                xc3x97                                  C                  p                                                                                    s                2                            +                              (                                  s                  xc3x97                                                            R                      mr                                                              2                      xc3x97                                              L                        p                                                                                            )                            +                              1                                  2                  xc3x97                                      L                    p                                    xc3x97                                      C                    p                                                                                xc3x97          Δ          ⁢                      xe2x80x83                    ⁢                      V            S                                                        xe2x80x83                ⁢                  EQ          ⁡                      (            7            )                              
According to EQ(7), the frequency response of Vout/xcex94Vs is not only determined by the preamplifier frequency response, but it is exhibiting an additional second-order system response given by the characteristic xcfx89n and Q of a second-order system.
The present trend in the industry is to use small values of Rmr. Unfortunately, as Rmr is reduced, the Q given by EQ(5) increases. This effect shows up as an increasing magnitude peaking at about the bandwidth edge of the Vout/xcex94Vs frequency response, as can be seen in the gain vs. frequency curve of FIG. 5.
One conventional technique of reducing this peaking is to add a differential capacitor array inside one gain stage 12 of the preamplifier 82, as shown in FIG. 6, to create a pole to suppress the peaking. As shown in FIG. 6, the preamplifier gain-stage 12 is a differential amplifier with emitter degeneration. Capacitors 58 and 60, of value CBWR, are connected across the load resistors 62 and 64 to provide poles to compensate the frequency peaks shown in FIG. 5. The pole frequency is given by                     Freq        =                  1                      2            xc3x97            π            xc3x97                          R              L                        xc3x97                          C              BWR                                                          EQ        ⁡                  (          8          )                    
and can be varied by varying CBBR to tailor to different Rmr values. For example, smaller Rmr (and thus more peaking) requires a lower frequency pole. This peak-suppression method is commonly known as the xe2x80x9cBandwidth Reduction Technique.xe2x80x9d
An undesirable side effect of this Bandwidth Reduction Technique is the presence of some frequency-response drooping at some frequencies below the passband edge. Ideally, the passband xe2x80x9cripplexe2x80x9d should be minimal and well below xc2x11 dB. The drooping becomes worse as more CBWR is employed to reduce larger peaking. This phenomenon can be seen in FIG. 5 as CBWR increases, as denoted by arrow 66.
What is needed, therefore, is a compensation technique that can reduce the peaking, with reduced frequency-response drooping at some frequencies below the passband edge.
In light of the above, therefore, it is an object of the invention to provide a compensation technique that can reduce the frequency response peaking of a magneto-resistive head, with reduced frequency-response drooping at some frequencies below the passband edge.
Thus, according to a broad aspect of the invention, a method is presented for reducing a frequency response peaking of a magneto-resistive head. The method includes providing an amplifier stage to amplify a signal that varies in response to an electrical characteristic of the head, and creating at least two poles in a frequency response of the amplifier to compensate for the frequency response peaking. Preferably, the poles have substantially identical pole locations. In a preferred embodiment, the amplifier stage comprises at least a pair of capacitor compensated differential amplifiers, and in another, the amplifier stage comprises three capacitor compensated differential amplifiers.
In accordance with another broad aspect of the invention, an amplifier is provided for use in amplifying a signal from a magneto-resistive head. The amplifier includes at least two gain stages connected to receive the signal representing changes in the magneto-resistive head. Capacitors are operatively connected within the gain stages to produce at least a pair of poles in a frequency response of the gain stages. Preferably, the poles have substantially identical pole locations. In a preferred embodiment, the capacitors are connected to produce two poles in the frequency response of the gain stages, and in another preferred embodiment, the capacitors are connected to produce three poles in the frequency response of the gain stages.
In yet another broad aspect of the invention, a mass data storage device system is provided which has a moving media that contains signals encoded in oriented magnetic domains. The system has a magneto-resistive head positioned in proximity to the moving media, wherein an electrical characteristic of the head, such as its resistance, is changed in accordance with the oriented magnetic domains. An amplifier stage is connected to sense the change in the electrical characteristic of the head, and capacitors are operatively connected within the amplifier stage to produce at least a pair of poles in a frequency response of the amplifier stage. In a preferred embodiment, the electrical characteristic of the head has a second order frequency response that is reduced by the pair of poles established by the capacitors. The amplifier stage may include, for example, at least two differential amplifiers, a first of the differential amplifiers being connected to sense a voltage across the magneto-resistive head, and the capacitors may include at least a pair of capacitors each associated with a respective one of the differential amplifiers to produce a pair of poles, in one embodiment, and three poles in another embodiment, in the frequency response of the amplifier stage.